Oh, the things humans will do to get their grubby hands on gold—a metal mostly prized for its ornamental use, hoarded in bank vaults and jewelry boxes, though we've arbitrarily decided it's worth, uh, its weight in gold.* The deepest gold mine in the world is Mponeng, a 2.5-mile hole in the ground in South Africa. A whole underground city—lightless and lawless—lives inside the mine.

Every day, 4,000 workers descend into the mine through elevators—or, as they're called in mining parlance, cages. These triple-decked cages fit 120 people at a time, and the first 1.6-mile shaft takes only 6 minutes to descend. A second shaft takes workers deeper down, and the last part is only accessible by foot or vehicle.

The rock is so hot underground that ice has to be pumped down to cool the tunnels.

Because temperatures increase the closer we get to the earth's core, the rock faces in the mine can get as hot as 140º F. "You can imagine what it's like to crawl into a cavity there," Hart said to NPR. "It's like crawling into a pizza oven."

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To keep those super-temperatures from becoming deadly, an ice-slurry mixed with salt is pumped down from the surface; huge fans then blow air over the ice, forming a controlled cold-air system within the mine—its own internal weather system. The above-ground ice-making plant goes through 6,000 tons of ice a day. Ultimately, this means that many tunnels can be kept at an almost bearable 85 degrees.

At least 10% of the gold in South African mines is stolen. Criminal syndicates help illegal "ghost" miners sneak into the mineshaft, where they then hide out for months at a time, turning ghostly from the lack of sunlight. Security guards also tend to let these ghost miners be: the illegal miners are often armed with AK-47s and beer bottle grenades, and it's all too easy to hear someone coming from far off in the mine. The mine is so big, it's difficult to police anyways.

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There's also a whole, well, underground economy where legal miners help out their illegal brethren. Since bread, for example, costs twelve time as much in the illegal economy, packing some extra lunch can get you much more than lunch money.

Gold is so expensive, the mine only needs to extract 0.35 ounces from a ton of rock to be profitable.

The world's loneliest ecosystem was discovered in Mponeng.

The rod-shaped bacterium Desulforudis audaxviator lives alone in the dark, hot waters of Mponeng; it is the sole member of the only single-species ecosystem discovered. All life forms need basic nutrients like carbon, hydrogen, nitrogen, and, usually, different bacteria pull different nutrients from the environment to form an ecosystem. But D. audaxviator is self-reliant, capable of producing everything itself. Scientists think the bacterium has not seen the surface of the earth in millions of years.

Bacteria that live in remote places like the depths of the Mponeng mine are called extremophiles for their ability to withstand seemingly impossible conditions. When motivated by money—out of greed or basic economic necessity—humans can do the same, it seems. [National Public RadioviaNature]

*Update: This article originally called gold "mostly useless," which was a reference to the fact that most gold is not used at all except for in cases of personal ornamentation or financial investment. Approximately 12% of the world's gold stocks are used in electronics and industry.